The present invention relates generally to an optical recording and reproducing system which optically records and reproduces information onto and from an optical recording medium such as an optical card or the like, and more particularly to an improved servo control technique for for the optical recording and reproducing system in which the position of an objective lens provided in an optical head for focusing light beam onto the information recording surface of an optical recording medium is servo-controlled to control tracking and focusing deviations of light spot on the information recording surface.
Optical recording and reproducing systems are conventionally known which record and reproduce information onto and from a card-type optical recording medium (namely, optical card) by moving the optical card relative to the optical axis of optical energy such as a laser beam. With the developments and wide spread of computers etc., wide spread of optical cards have been highly expected in recent years because they are portable and safe and provide a relatively large storage capacity in spite of their small size, and so a variety of applications of the optical cards have been contemplated, such as application as recording media for patients' diagnosis in medical organizations.
A typical example of a known optical card is shown in FIGS. 4 and 5, of which FIG. 4 is a plan view of the known optical card 11 and FIG. 5 shows a section "A" in enlarged scale of the optical card 11 of FIG. 4. In these figures, reference numeral 12 denotes a recording/reproducing area, 13 denotes guide tracks, and 14 denotes a data track. 0n the recording/reproducing area is formed a recording layer that is for example made of silver chloride photographic material as base material. By irradiating laser light of a suitable energy level from an optical head onto the recording layer, an optical information unit called a "pit" is formed in the data track 14. The irradiated position on the recording layer of the laser light (laser light spot) is varied by moving the optical card 11 relative to the optical head in the X-axis direction (direction parallel to the data and guide tracks of the optical card 11), so that a series of pits can be formed in a desired arrangement corresponding to desired digital information. Thus, recording and reproduction of desired digital information are performed by writing and reading the pit rows onto and from the recording layer of the optical card 11.
In such a case, to form pit rows in the data track 14 of the optical card 11, an approach is generally employed which uses a drive mechanism such as a linear motor to move the optical card 11 relative to the optical head. However, due to a limited operational accuracy of the drive mechanism, the prior approach can not prevent occurrence of mechanical position error, due to which pits can not be formed accurately in the middle of the data track 14 located between the guide tracks 13. This presents a significant inconvenience that desired information can not be recorded or reproduced accurately.
In order to prevent the above-mentioned inconvenience, it is absolutely necessary to perform the pit recording and reproduction with the light spot accurately positioned in the middle between the two guide tracks 13. To this end, automatic tracking control has been conventionally performed in an attempt to compensate for any mechanical position error caused.
This automatic tracking control is generally performed on the basis of the so-called "three-beam method", in which three laser beams spaced apart from each other by a predetermined distance are irradiated from the optical head in such a manner that the central laser beam corresponds in to the data track 14 as a read/write beam and the two laser beams on both sides of the central beam (side laser beams) correspond to the guide tracks 13 on both sides of the data track 14 as tracking beams. Namely, the three-beam method measures the respective reflected lights of the two side laser beams from the optical card 11 so as to servo-control the irradiated light spot positions in such a manner that the tracking beams accurately correspond to the guide tracks 13 in predetermined positional relations and thus the central read/write beam is allowed to be always accurately positioned in a predetermined middle part of the data track 14.
Further, since it is necessary to constantly have laser light beam(s) stably focused on the recording layer of the optical card 11, automatic focusing control has also been conventionally performed in the art.
The above-mentioned automatic tracking and focusing control operations are respectively performed by minutely driving the objective lens of the optical head, via electromagnetic force applied via a tracking coil and a focusing coil, in the Y-axis direction (i.e., direction transverse to the data and guide tracks of the optical card 11) and in the Z-axis direction (i.e., direction perpendicular to the recording/reproducing surface of the optical card 11). The objective lens serves to focus the laser beam irradiated from the optical head onto the recording layer of the optical card 11 so as to form a light spot (three light spots in the case of the above-mentioned three-beam method) on the recording layer.
FIGS. 6 and 7 show examples of the prior art servo control systems used for the automatic tracking and focusing control.
More specifically, FIGS. 6 and 7 show examples of conventional tracking control circuits 30a and 30b, respectively, for performing the above-mentioned automatic tracking control; FIG. 6 illustrates the basic arrangement for the automatic tracking control, and FIG. 7 illustrates an improved technique as disclosed in Japanese Patent Publication No. HEI 5-47895.
First, the tracking control circuit 30a of FIG. 6 will be described. A light detector 31 having a pair of light receiving elements 31a and 31b disposed in a predetermined arrangement is mounted within the optical head for photoelectrically converting the reflection from the optical card and input via the objective lens. The above-mentioned two tracking beams of the reflection input to the light detector 31 are received by the light receiving elements 31a and 31b, respectively, so that received-light signals responsive to possible tracking error are output from the light receiving elements 31a and 31b. The output signals from the light receiving elements 31a and 31b are then fed to a differential amplifier 32 which in turn outputs a difference signal between the fed signals and are also fed to a summing amplifier 33 which in turn outputs a sum signal of the fed signals. This sum signal is provided to a division circuit 34 as a denominator signal A. The difference signal is, on the other hand, provided to the division circuit 34 as a numerator and is divided by the sum signal to be normalized.
For example, when the tracking light spots are accurately positioned on the respective guide tracks 13 of the optical card 11, the difference signal has a value of zero or near zero, while the sum signal has a greatest value. Accordingly, the output from the division circuit 34 has a value of zero or near zero, which indicates that the laser light spots are located at a suitable position with respect to the guide tracks 13.
When the tracking laser light spots are deviated from the guide tracks 13, however, the difference signal has a value greater or smaller than zero so that the output from the division circuit 34 has a value greater than zero. In this way, the division circuit 34 outputs a signal corresponding to deviation amounts of the laser light spots from the guide tracks 13 (i.e., tracking error). To provide enhanced stability of the control loop, the output signal from the division circuit 34 is subjected to "phase-advancing compensation" by a phase compensation circuit 35. The signal having undergone the phase-advancing compensation is converted by a drive circuit 36 into electric current, which is applied to a tracking coil 20 so as to drive the objective lens in the Y-axis direction (direction transverse to the data and guide tracks of the optical card 11). Thus, the objective lens is driven in dependence on the deviation amounts of the laser light spots from the guide tracks 13.
The purpose of normalizing the difference signal by means of the division circuit 34 as previously noted is to keep constant the servo loop gain of the tracking control circuit in order to guarantee accurate information recording onto the optical card 11. For example, to comparatively consider a possible variation between the amounts or power levels of the laser light generated during the recording and reproduction operations, the laser light power during the recording is about 1.5 mW, which is about one hundred times as large as the laser light power during the reproduction of about 0.1 to 0.4 mW. Therefore, it is vitally important to maintain a constant servo loop gain without being influenced by such variation in the laser light amount.
The tracking control circuit 30b of FIG. 7 is designed to maintain a constant servo loop gain of the tracking control circuit without using the division circuit as shown in FIG. 6. Light detection signals output from light receiving elements 31a and 31b are fed to variable-gain amplifiers 37a and 37b, which are constructed in such a manner to stepwise switch their amplification factors in response to instructions given from a control circuit 38. Sample and hold circuits 39a and 39b sample and hold the respective output signals from the variable-gain amplifiers 37a and 37b. Further, a subtraction circuit 40 performs subtraction between the output signals of the sample and hold circuits 39a and 39b to thereby provide a tracking error signal. The tracking error signal provided from the subtraction circuit 40 is subjected to phase-advancing compensation by a phase compensation circuit 35 as in the case of FIG. 6 and is applied to a drive circuit 36, which in turn drives the tracking coil 20.
FIG. 8 is a signal timing chart explanatory of an exemplary operation of the prior art technique as shown in FIG. 7. Item (a) of FIG. 8 indicates signal representing information to be recorded onto the optical card 11 (recording information signal), item (b) indicates variations in the laser light power modulated by the recording information signal, and item (c) indicates outputs from the subtraction circuit 40. As shown in item (a) of FIG. 8, the recording information signal varies between values "1" and "0". In response to the variation in the recording information signal, the generated laser light power varies between levels "PL" and "PH" as shown in item (b). A pit is formed in the optical card 11 when the laser light power is at high level "PH", while no pit is formed in the optical card 11 when the laser light power is at low level "PL". Thus, the outputs of the light receiving elements 31a and 31b vary in accordance with the variations in the laser light power (between levels "PL" and "PH"). Here, in order to maintain a constant open-loop gain of the tracking servo, the respective gains of the variable-gain amplifiers 37a and 37b are changed. Additionally, in order to eliminate discrepancies in the rise and/or fall of signals in the servo loop which would be caused by sudden changes in the light amount as shown in item (b), an approach is taken in accordance with which signals in the servo loop are sampled and held by the sample and hold circuits 39a and 39b before the light amount change begins, and the thus-held signals are released after the light amount change.
However, the above-mentioned prior art arrangements have the following shortcomings.
First, the FIG. 6 arrangement tends to be expensive because it requires complicated analog processes using the division circuit 34, and the arrangement is susceptible to thermal drift due to the use of analog circuitry, thus causing frequent tracking errors.
In contrast, the FIG. 7 arrangement provides a solution to the problems of the above-mentioned FIG. 6 arrangement using the division circuit, by suitably changing the gains of the variable-gain amplifiers 37a and 37b to maintain a constant loop gain against the laser light power variations. Nevertheless, this arrangement presents new problems that the frequency response is poor due to the necessity to change the gains of the variable-gain amplifiers 37a and 37b, and the loop gain undesirably varies because the reflection factors on the recording layer may differ among different optical cards. Such conditions are shown in item (c) of FIG. 8. Assuming that the gain for an optical card of suitable reflectivity is Va, the gain for an optical recording medium of higher reflectivity will increase to Vb, and the gain for an optical recording medium of lower reflectivity will decrease to Vc. Under such conditions, accurate servo control can not be achieved and tracking errors would result.
The automatic focusing control employs servo control circuitry similar to the above-mentioned and hence would encounter the same kinds of problems.
In recent years, an approach has been taken to increase the recording density of optical cards in order to provide increased storage capacity. In the light of such increased density of optical cards, enhanced accuracy and stability of the tracking and focusing control in particular are highly demanded of the optical card recording and reproducing devices. Therefore, it is extremely important to overcome the above-mentioned problems.